1. Field of the Invention
The invention relates to an electroless plating bath control apparatus and somewhat more particularly to such a control apparatus for electroless plating of copper wherein at least formaldehyde, sodium hydroxide and a copper salt comprise the main bath conponents and their respective concentration is controllable to relatively constant values, with the copper-ion concentration being colorimetrically defined and the formaldehyde-ion and hydroxyl-ion concentrations being titrimetrically defined.
2. Prior Art
In electroless plating or precipitation of, for example, copper from a suitable chemical copper bath, the concentration of main components of such bath must be analyzed and controlled or replenished as needed so that the precipitation conditions remain substantially constant and substantially faultless copper layers are attained.
A control apparatus for use with an electroless copper plating bath is described in U.S. Pat. No. 4,096,301. In this apparatus, a bath sample is continuously removed from the copper plating bath. A standardized acid of a select concentration and amount is likewise continuously added to the bath sample so that a final acid value, defined in terms of plating potential, is achieved. After appropriate mixing, the acidified bath sample passes through a pH analyzing station wherein the actual pH value is measured and compared with a predefined rated value. Given a deviation from such rated value, sodium hydroxide solution is added to the copper plating bath is accordance with such deviation. Thereafter, the so-processed bath sample is passed through a colorimetric station wherein the copper ion concentration is analyzed or monitored and, given a deviation from a rated value, an appropriate amount of fresh copper salt solution is added to th copper bath so as to replenish the concentration of copper ion therein in an amount corresponding to the observed deviation. After passing through the colorimetric station, sodium sulfide is constantly added to the resultant bath sample and, after appropriate mixing, this buffered bath sample is passed to a further pH analyzing station where the pH value of the bath sample is again determined and the difference between the now-measured value and the previously measured pH value is determined. This difference in pH value is utilized as an indirect measure of the formaldehyde concentration in the plating bath. Again, given a deviation from a rated value, an appropriate amount of formaldehyde is added to the plating bath.
A somewhat similar control system for use with an electroless copper plating bath is described in German Offenlegungsschrift 27 51 104. In this system, a bath sample is likewise continuously removed from a chemical copper bath and introduced into a chamber in which a "precipitation" electrode is positioned. Adjacent to this chamber, a second chamber is positioned with a "comparison" electrode therein, which together with the precipitation electrode functions to determine a so-called "mixing potential". After the determination of such mixing potential, the resultant bath sample is passed, via a heat-exchanger means, to a pH analyzing station and a colorimetric station. The individual bath components are then replenished as needed as a function of the mixing potential.
In the above described known plating bath control apparatuses, the concentration of the individual conmponents are not positively or absolutely determined or displayed and instead such concentrations are only determined relative to predetermined rated values. The absolute concentration of the individual components thus is never known. Further, the determination of pH value is also problematical since this value cannot be held constant over extended time periods due to electrode drift. Accordingly, an occasional re-calibration is unavoidably necessary.